Carbon dioxide -- not water -- triggers explosive basaltic volcanoes

Geoscientists have long thought that water -- along with shallow magma stored in Earth's crust -- drives volcanoes to erupt. Now, thanks to newly developed research tools at Cornell, scientists have learned that gaseous carbon dioxide can trigger explosive eruptions.

A new model suggests that basaltic volcanoes, typically located on the interior of tectonic plates, are fed by a deep magma within the mantle, stored about 20 to 30 kilometers below Earth's surface.

The research, which offers a clearer picture of our planet's deep internal dynamics and composition, with implications for improving volcanic-hazards planning, will publish August 7, 2023 at 3:00pm ET in the Proceedings of the National Academy of Sciences.

"We used to think all the action happened in the crust," said senior author Esteban Gazel, the Charles N. Mellowes Professor in Engineering in the Department of Earth and Atmospheric Sciences, in Cornell Engineering. "Our data implies the magma comes directly from the mantle -- passing fast through the crust - driven by the exsolution (the process phase of separating gas from liquid) of carbon dioxide.

"This completely changes the paradigm of how these eruptions happen," Gazel said. "All volcanic models had been dominated by water as the main eruption driver, but water has little to do with these volcanoes. It's carbon dioxide that brings this magma from the deep Earth."

About four years ago, Gazel and Charlotte DeVitre, Ph.D. '22, now a postdoctoral researcher at University of California, Berkeley, developed a high-precision carbon dioxide densimeter (which measures density in a tiny vessel) for Raman spectroscopy (a device that examines scattered photons through a microscope).

The natural samples -- microscopic-sized carbon dioxide rich bubbles trapped in crystals emanating from the volcanic eruption -- are then measured via Raman and quantified applying the newly developed densimeter. Essentially, the scientists are examining a microscopic time capsule to provide a history of the magma. This new technique is critical for near real-time precise estimations of magma storage, tested during the 2021 eruption in Las Palmas, in the Canary Islands by Gazel's group.

Further, the scientists developed methods to assess the effect of laser heating on carbon-dioxide rich inclusions (found swathed in the crystals), and to accurately assess melt inclusion and bubble volumes. They also developed an experimental reheating method to increase accuracy and properly account for carbon dioxide trapped as carbonate crystals inside the bubbles.

"The method of development and instrument design were challenging, especially during the height of the pandemic," Gazel said.

Using these new tools, the scientists scrutinized volcanic deposits from the Fogo volcano in Cabo Verde, west of Senegal in the Atlantic Ocean. They found a high concentration of volatiles in the micro-sized melt inclusions encased within the magnesium-iron silicate crystals. The higher amount of carbon dioxide enclosed in the crystals suggested that the magma was stored tens of kilometers below the surface -- within the Earth's mantle.

The group also discovered that this process is connected to the deep mantle source that supply these volcanoes.

This implies that eruptions such as Fogo's volcanic flareups start and are fed from the mantle, effectively bypassing storage in the Earth's crust and driven by deep carbon dioxide, according to the paper.

"These magmas have extremely low viscosities and come directly from the mantle," DeVitre said. "So here, viscosity and water cannot play the common roles that they do in shallower and/or more silicic (rich in silica) volcanic systems. Rather at Fogo volcano the magma must be driven up fast by the carbon dioxide and this likely plays a significant role in its explosive behavior. This is a major step in our understanding of the controls on basaltic explosivity."

Comprehending magma storage helps best prepare society for future eruptions, said Gazel, who is also a faculty fellow at the Cornell Atkinson Center for Sustainability.

"As deep magma storage will not be detected by ground deformation until the melt is close to surface," he said, "this has important repercussions to our understanding of volcanic hazards. We need to understand the drivers of these eruptions. The only way to see these processes now is by observing earthquakes, but earthquakes don't tell you exactly what's happening."

Said Gazel: "With precise measurements that tell us where eruptions start, where magmas melt and where they are stored -- and what triggers the eruption -- we can develop a much better plan for future eruptions."

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Geologists challenge conventional view of Earth's continental history, stability with new study

The seemingly stable regions of the Earth's continental plates -- the so-called stable cratons -- have suffered repetitive deformation below their crust since their formation in the remote past, according to new research from the University of Illinois Urbana-Champaign. This hypothesis defies decades of conventional plate tectonics theory and begs to answer why most cratons have remained structurally stable while their underbellies have experienced significant change.

In a study led by Illinois geology professor Lijun Liu, researchers used previously collected density data from the Earth's uppermost rigid layers of crust and mantle -- known as the lithosphere -- to examine the relationship between craton surface topography and the thickness of their underlying lithosphere layer.

The results of the study are published in the journal Nature Geosciences.

The lack of deformation within the cratons since their formation makes them the longest-lived tectonic units on Earth -- surviving supercontinent cycles like the formation and breakup of the supercontinent Pangea, as well as the lesser-known and more ancient supercontinent Rodina, the study reports.

"It is generally accepted that the cratons are protected by their thick underlying mantle roots, or keels, which are believed to be buoyant and strong and thus stable over time," Lui said.

Several recent papers from Liu's research group directly challenge this wisdom by showing that these mantle keels are actually quite dense.

In a 2022 study, the team demonstrated that the traditional view of buoyant craton keels implies that most of the Earth's cratons would be sitting about 3 kilometers above the sea surface, while in reality, their elevation is only a few 100 meters. This requires the lithospheric mantle below the crust to be of high enough density to pull the surface down by about 2 kilometers, Liu said.

In another study, the team used gravity field measurements to pinpoint the density structure of the craton keels to find that the lower portion of the mantle keel is most likely where the high-density material resides, implying a depth-increasing density profile below the cratons.

In the new paper, the team shows that the lower portion of the mantle keel that has a high density and tends to repeatedly peel away from the lithosphere above when mantle upwellings, called plumes, initiate supercontinent breakup. The peeled-off -- or delaminated -- keels could return to the base of the lithosphere after they warm up inside the hot mantle.

"The whole process is like what happens in a lava lamp, where the cool material near the surface sinks and the warm material near the bottom rises," Liu said.

This deformation history is expressed in some of the more puzzling geophysical properties observed in the lithosphere, the study reports.

"For example, the repetitive vertical deformation of the lower half of the mantle keel allows the seismic waves that vibrate the rock vertically to travel faster, compared to the upper half of the keel, which experienced less vertical deformation," Liu said.

The team also determined that mantle delamination will cause the craton surface to rise, leading to erosion.

"This is reflected in the strong dependence of crustal thickness on lithospheric thickness, an observation never made before this study," Liu said. "In particular, there are two major uplift and erosion events in the past, when supercontinents Rodinia and Pangea each separated, the former causing what is known as the Great Unconformity -- a feature in the Earth's rock record shows no evidence of new deposition, only deep craton erosion. This is the reason why we see pieces of ancient lower crust exposed at the craton's surface today."

With the help of numerical simulations, the team said that this episodic deformation style of the lower craton keels is how the craton crusts survived the long geological history.

"We believe this newly hypothesized lifestyle of cratons will significantly change people's view on how continents evolve and how plate tectonics operate on Earth," Liu said.

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New clues about the rise of Earth's continents

Continents are part of what makes Earth uniquely habitable for life among the planets of the solar system, yet surprisingly little is understood about what gave rise to these huge pieces of the planet's crust and their special properties. New research from Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian's National Museum of Natural History, and lead study author Megan Holycross, formerly a Peter Buck Fellow and National Science Foundation Fellow at the museum and now an assistant professor at Cornell University, deepens the understanding of Earth's crust by testing and ultimately eliminating one popular hypothesis about why continental crust is lower in iron and more oxidized compared to oceanic crust. The iron-poor composition of continental crust is a major reason why vast portions of the Earth's surface stand above sea level as dry land, making terrestrial life possible today.

The study, published today in Science, uses laboratory experiments to show that the iron-depleted, oxidized chemistry typical of Earth's continental crust likely did not come from crystallization of the mineral garnet, as a popular explanation proposed in 2018.

The building blocks of new continental crust issue forth from the depths of the Earth at what are known as continental arc volcanoes, which are found at subduction zones where an oceanic plate dives beneath a continental plate. In the garnet explanation for continental crust's iron-depleted and oxidized state, the crystallization of garnet in the magmas beneath these continental arc volcanoes removes non-oxidized (reduced or ferrous, as it is known among scientists) iron from the terrestrial plates, simultaneously depleting the molten magma of iron and leaving it more oxidized.

One of the key consequences of Earth's continental crust's low iron content relative to oceanic crust is that it makes the continents less dense and more buoyant, causing the continental plates to sit higher atop the planet's mantle than oceanic plates. This discrepancy in density and buoyancy is a major reason that the continents feature dry land while oceanic crusts are underwater, as well as why continental plates always come out on top when they meet oceanic plates at subduction zones.

The garnet explanation for the iron depletion and oxidation in continental arc magmas was compelling, but Cottrell said one aspect of it did not sit right with her.

"You need high pressures to make garnet stable, and you find this low-iron magma at places where crust isn't that thick and so the pressure isn't super high," she said.

In 2018, Cottrell and her colleagues set about finding a way to test whether the crystallization of garnet deep beneath these arc volcanoes is indeed essential to the process of creating continental crust as is understood. To accomplish this, Cottrell and Holycross had to find ways to replicate the intense heat and pressure of the Earth's crust in the lab, and then develop techniques sensitive enough to measure not just how much iron was present, but to differentiate whether that iron was oxidized.

To recreate the massive pressure and heat found beneath continental arc volcanoes, the team used what are called piston-cylinder presses in the museum's High-Pressure Laboratory and at Cornell. A hydraulic piston-cylinder press is about the size of a mini fridge and is mostly made of incredibly thick and strong steel and tungsten carbide. Force applied by a large hydraulic ram results in very high pressures on tiny rock samples, about a cubic millimeter in size. The assembly consists of electrical and thermal insulators surrounding the rock sample, as well as a cylindrical furnace. The combination of the piston-cylinder press and heating assembly allows for experiments that can attain the very high pressures and temperatures found under volcanoes.

In 13 different experiments, Cottrell and Holycross grew samples of garnet from molten rock inside the piston-cylinder press under pressures and temperatures designed to simulate conditions inside magma chambers deep in Earth's crust. The pressures used in the experiments ranged from 1.5 to 3 gigapascals -- that is roughly 15,000 to 30,000 Earth atmospheres of pressure or 8,000 times more pressure than inside a can of soda. Temperatures ranged from 950 to 1,230 degrees Celsius, which is hot enough to melt rock.

Next, the team collected garnets from Smithsonian's National Rock Collection and from other researchers around the world. Crucially, this group of garnets had already been analyzed so their concentrations of oxidized and unoxidized iron were known.

Finally, the study authors took the materials from their experiments and those gathered from collections to the Advanced Photon Source at the U.S. Department of Energy's Argonne National Laboratory in Illinois. There the team used high-energy X-ray beams to conduct X-ray absorption spectroscopy, a technique that can tell scientists about the structure and composition of materials based on how they absorb X-rays. In this case, the researchers were looking into the concentrations of oxidized and unoxidized iron.

The samples with known ratios of oxidized and unoxidized iron provided a way to check and calibrate the team's X-ray absorption spectroscopy measurements and facilitated a comparison with the materials from their experiments.

The results of these tests revealed that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron-depletion and oxidation present in the magmas that are the building blocks of Earth's continental crust.

"These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron depleted," Cottrell said. "It's more likely that conditions in Earth's mantle below continental crust are setting these oxidized conditions."

Like so many results in science, the findings lead to more questions: "What is doing the oxidizing or iron depleting?" Cottrell asked. "If it's not garnet crystallization in the crust and it's something about how the magmas arrive from the mantle, then what is happening in the mantle? How did their compositions get modified?"

Cottrell said that these questions are hard to answer but that now the leading theory is that oxidized sulfur could be oxidizing the iron, something a current Peter Buck Fellow is investigating under her mentorship at the museum.

This study is an example of the kind of research that museum scientists will tackle under the museum's new Our Unique Planet initiative, a public-private partnership, which supports research into some of the most enduring and significant questions about what makes Earth special. Other research will investigate the source of Earth's liquid oceans and how minerals may have served as templates for life.

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Mars's crust more complex, evolved than previously thought

Early crust on Mars may be more complex than previously thought -- and it may even be similar to our own planet's original crust.

The Martian surface is uniformly basaltic, a product of billions of years of volcanism and flowing lava on the surface that eventually cooled. Because Mars did not undergo full-scale surface remodeling like the shifting of continents on Earth, scientists had thought Mars' crustal history was a relatively simple tale.

But in a new study, researchers found locations in the Red Planet's southern hemisphere with greater concentrations of silicon, a chemical element, than what would be expected in a purely basaltic setting. The silica concentration had been exposed by space rocks that slammed into Mars, excavating material that was embedded miles below the surface, and revealing a hidden past.

"There is more silica in the composition that makes the rocks not basalt, but what we call more evolved in composition," says Valerie Payré, assistant professor in the Department of Earth and Environmental Sciences at the University of Iowa and the study's corresponding author. "That tells us how the crust formed on Mars is definitely more complex than what we knew. So, it's more about understanding that process, and especially what it means for how Earth's crust first formed."

Scientists believe Mars formed about 4.5 billion years ago. Exactly how the Red Planet came into being is a mystery, but there are theories. One idea is that Mars formed via a titanic collision of rocks in space that, with its intense heat, spawned an entirely liquefied state, also known as a magma ocean. The magma ocean gradually cooled, the theory goes, yielding a crust, like a layer of skin, that would be singularly basaltic.

Another theory is that the magma ocean was not all-encompassing, and that parts of the first crust on Mars had a different origin, one that would show silica concentrations different from basaltic.

Payré and her research partners analyzed data gathered by the Mars Reconnaissance Orbiter for the planet's southern hemisphere, which previous research had indicated was the oldest region. The researchers found nine locations -- such as craters and fractures in the terrain -- that were rich in feldspar, a mineral associated with lava flows that are more silicic than basaltic.

"This was the first clue," Payré says. "It is because the terrains are feldspar-rich that we explored the silica concentrations there."

Feldspar had been found previously in other regions on Mars, but further analysis showed the chemical composition in those areas was more basaltic. That did not deter the researchers, who turned to another instrument, called THEMIS, which can detect silica concentrations through infrared wavelength reflections from the Martian surface. With data from THEMIS, the team determined the terrain at their chosen locations was more silicic than basaltic.

Adding further credence to their observations, meteorites such as Erg Chech 002, discovered in the Sahara and dating roughly to the birth of the solar system, show similar silicic and other mineral compositions that the team observed in the nine locations on Mars.

The researchers also dated the crust to about 4.2 billion years, which would make it the oldest crust found on Mars to date.

Payré says she was mildly surprised at the discovery.

"There have been rovers on the surface that have observed rocks that were more silicic than basaltic," she says. "So, there were ideas that the crust could be more silicic. But we never knew, and we still don't know, how the early crust was formed, or how old it is, so it's kind of a mystery still."

While Mars' crustal origin remains shrouded, Earth's crustal history is even less clear, as any vestiges of our planet's original crust have been long erased due to the shifting of continental plates for billions of years. Still, the finding may offer insights into Earth's origins.

"We don't know our planet's crust from the beginning; we don't even know when life first appeared," Payré says. "Many think the two could be related. So, understanding what the crust was like a long time ago could help us understand the whole evolution of our planet."

Read more at Science Daily

Laying geological groundwork for life on Earth

New research analyzing pieces of the most ancient rocks on the planet adds some of the sharpest evidence yet that Earth's crust was pushing and pulling in a manner similar to modern plate tectonics at least 3.25 billion years ago. The study also provides the earliest proof of when the planet's magnetic north and south poles swapped places.

The two results offer clues into how such geological changes may have resulted in an environment more conducive to the development of life on the planet.

The work, described in PNAS and led by Harvard geologists Alec Brenner and Roger Fu, focused on a portion of the Pilbara Craton in western Australia, one of the oldest and most stable pieces of the Earth's crust. Using novel techniques and equipment, the researchers show that some of the Earth's earliest surface was moving at a rate of 6.1 centimeters per year and 0.55 degrees every million years.

That speed more than doubles the rate the ancient crust was shown to be moving in a previous study by the same researchers. Both the speed and direction of this latitudinal drift leaves plate tectonics as the most logical and strongest explanations for it.

"There's a lot of work that seems to suggest that early in Earth's history plate tectonics wasn't actually the dominant way in which the planet's internal heat gets released as it is today through the shifting of plates," said Brenner, a Ph.D. candidate in the Graduate School of Arts and Sciences and member of Harvard's Paleomagnetics Lab. "This evidence lets us much more confidently rule out explanations that don't involve plate tectonics."

For example, the researchers can now argue against phenomena called "true polar wander" and "stagnant lid tectonics," which can both cause the Earth's surface to shift but aren't part of modern-style plate tectonics. The results lean more toward plate tectonic motion because the newly discovered higher rate of speed is inconsistent with aspects of the other two processes.

In the paper, the scientists also describe what's believed to be the oldest evidence of when Earth reversed its geomagnetic fields, meaning the magnetic North and South Pole flipped locations. This type of flip-flop is a common occurrence in Earth's geologic history with the pole's reversing 183 times in the last 83 million years and perhaps several hundred times in the past 160 million years, according to NASA.

The reversal tells a great deal about the planet's magnetic field 3.2 billion years ago. Key among these implications is that the magnetic field was likely stable and strong enough to keep solar winds from eroding the atmosphere. This insight, combined with the results on plate tectonics, offers clues to the conditions under which the earliest forms of life developed.

"It paints this picture of an early earth that was already really geodynamically mature," Brenner said. "It had a lot of the same sorts of dynamic processes that result in an Earth that has essentially more stable environmental and surface conditions, making it more feasible for life to evolve and develop."

Today, the Earth's outer shell consists of about 15 shifting blocks of crust, or plates, which hold the planet's continents and oceans. Over eons the plates drifted into each other and apart, forming new continents and mountains and exposing new rocks to the atmosphere, which led to chemical reactions that stabilized Earth's surface temperature over billions of years.

Evidence of when plate tectonics started is hard to come by because the oldest pieces of crust are thrust into the interior mantle, never to resurface. Only 5 percent of all rocks on Earth are older than 2.5 billion years old, and no rock is older than about 4 billion years.

Overall, the study adds to growing research that tectonic movement occurred relatively early in Earth's 4.5-billion-year history and that early forms of life came about in a more moderate environment. Members of the project revisited the Pilbara Craton in 2018, which stretches about 300 miles across. They drilled into the primordial and thick slab of crust there to collect samples that, back in Cambridge, were analyzed for their magnetic history.

Using magnetometers, demagnetizing equipment, and the Quantum Diamond Microscope -- which images the magnetic fields of a sample and precisely identifies the nature of the magnetized particles -- the researchers created a suite of new techniques for determining the age and way the samples became magnetized. This allows the researchers to determine how, when, and which direction the crust shifted as well as the magnetic influence coming from Earth's geomagnetic poles.

The Quantum Diamond Microscope was developed in a collaboration between Harvard researchers in the Departments of Earth and Planetary Sciences (EPS) and of Physics.

For future studies, Fu and Brenner plan keep their focus on the Pilbara Craton while also looking beyond it to other ancient crusts around the world. They hope to find older evidence of modern-like plate motion and when the Earth's magnetic poles flipped.

Read more at Science Daily

Volcanic super eruptions are millions of years in the making -- followed by swift surge

Researchers at the University of Bristol and Scottish Universities Environmental Research Centre have discovered that super-eruptions occur when huge accumulations of magma deep in the Earth's crust, formed over millions of years, move rapidly to the surface disrupting pre-existing rock.

Using a model for crustal flow, an international team of scientists were able to show that pre-existing plutons -- a body of intrusive rock made from solidified magna or lava -- were formed over a few million years prior to four known gigantic super eruptions and that the disruption of these plutons by newly emplaced magmas took place extraordinarily rapidly. While the magma supplying super eruptions takes place over a prolonged period of time, the magma disrupts the crust and then erupts in just a few decades.

The findings, published today in Nature, explain these extreme differences in time ranges for magma generation and eruption by flow of hot but solid crust in response to ascent of the magma, accounting for the infrequency of these eruptions and their huge volumes.

Professor Steve Sparks of Bristol's School of Earth Sciences explained: "The longevity of plutonic and related volcanic systems contrasts with short timescales to assemble shallow magma chambers prior to large-magnitude eruptions of molten rock. Crystals formed from earlier magma pulses, entrained within erupting magmas are stored at temperatures near or below the solidus for long periods prior to eruption and commonly have very short residence in host magmas for just decades or less."

This study casts doubt on the interpretation of prolonged storage of old crystals at temperatures high enough for some molten rocks to be present and indicates the crystals derived from previously emplaced and completely solidified plutons (granites).

Scientists have known that volcanic super-eruptions eject crystals derived from older rocks. However, before this, they were widely thought to have originated in hot environments above the melting points of rock. Previous studies that show the magma chambers for super-eruptions form very rapidly but there was no convincing explanation for this rapid process. While modelling suggested that super-volcanic eruptions would need to be preceded by very long periods of granite pluton emplacement in the upper crust, evidence for this inference was largely lacking.

Prof Sparks added: "By studying of the age and character of the tiny crystals erupted with molten rock, we can help understand how such eruptions happen.

"The research provides an advance in understanding the geological circumstances that enable super eruptions to take place. This will help identify volcanoes that have potential for future super-eruptions."

Such eruptions are very rare and Bristol scientists estimate only one of these types of eruptions occur on earth every 20,000 years. However such eruptions are highly destructive locally and can create global scale severe climate change that would have catastrophic consequences.

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Sampling the deep graveyard of Earth's earliest crust

In an international collaboration, Earth scientists at the University of Cologne and Freie Universität Berlin discovered that some magmas on Earth, which made their way through the deep terrestrial mantle and erupted at Earth's surface, originate from mantle portions that contain remnants of Earth's earliest crust. This ancient material must have been buried in a 'graveyard' of old and cold crust more than 4 billion years ago and survived since then, maybe since the giant impact event forming the Moon.

This finding is unexpected because the plate tectonic regime of our planet progressively recycles crustal material via large-scale mantle convection at much smaller time scales. Therefore, it has been assumed that vestiges of early geological processes on Earth can only be found as analogues, on other terrestrial planets (Mercury, Venus, and Mars), asteroids, or the Moon. However, according to their study 'Long-term preservation of Hadean protocrust in Earth's mantle', which has recently appeared in the Proceedings of the National Academy of Sciences (PNAS), magmatic rocks that erupted throughout Earth's history can still carry signatures that provide detailed information about the nature of the first crust, its long-term preservation in a graveyard in the lower-most mantle, and its resurrection via younger volcanism.

For their study, the geologists investigated up to 3.55 billion years old rocks from southern Africa. The analysis of these rocks revealed small anomalies in the isotope composition of the element tungsten (W). The origin of these isotope anomalies, namely the relative abundance of 182W, relates to geological processes that must have occurred immediately after the formation of the Earth more than 4.5 billion years ago.

Model calculations by the authors show that the observed 182W isotope patterns are best explained by the recycling of Earth's earliest crust into mantle material that ascends via plumes from the lower mantle to generate lavas erupting at Earth's surface. Intriguingly, the study shows that similar isotope patterns can be observed in distinct types of modern volcanic rocks (ocean island basalts), which demonstrates that Earth's earliest crust is still buried in the lowermost mantle.

'We assume that the lower layers of the crust -- or the roots of the primordial continents -- became heavier than their surroundings due to a geological maturation process and therefore sank into the Earth's underlying mantle. Similar to a lava lamp,' geochemist Dr Jonas Tusch from the University of Cologne's Institute of Geology and Mineralogy remarked. 'This fascinating insight provides a geochemical fingerprint of the young Earth, allowing us to better understand how large continents formed over the history of our planet. It also explains how our current, oxygen-rich atmosphere evolved -- setting the stage for the origin of complex life,' Dr Elis Hoffmann of Freie Universität Berlin added.

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Earliest geochemical evidence of plate tectonics found in 3.8-billion-year-old crystal

A handful of ancient zircon crystals found in South Africa hold the oldest evidence of subduction, a key element of plate tectonics, according to a new study published today in AGU Advances, AGU's journal for high-impact, open-access research and commentary across the Earth and space sciences.

These rare time capsules from Earth's youth point to a transition around 3.8 billion years ago from a long-lived, stable rock surface to the active processes that shape our planet today, providing a new clue in a hot debate about when plate tectonics was set in motion.

Earth's crust and the top layer of mantle just under it are broken up into rigid plates that move slowly on top of viscous but mobile lower layers of mantle rock. Heat from Earth's core drives this slow but inexorable motion, responsible for volcanoes, earthquakes, and the uplift of mountain ranges.

Estimates for when this process revved up and modern crust formed range from over 4 billion years ago to just 800 million years ago. Uncertainty arises because the geologic record from Earth's youth is sparse, due to the surface recycling effect of plate tectonics itself. Almost nothing remains from the Hadean Eon, Earth's first 500 million years.

"The Hadean Earth is this big mystery box," said Nadja Drabon, a geologist at Harvard University and the lead author of the new study.

Tiny time capsules

In an exciting step forward in solving this mystery, in 2018 Drabon and her colleagues unearthed a chronological series of 33 microscopic zircon crystals from a rare, ancient block of crust in the Barberton Greenstone Belt in South Africa, that formed at different times over a critical 800-million-year span from 4.15 to 3.3 billion years ago.

Zircon is a relatively common accessory mineral in Earth's crust, but ancient representatives from the Hadean Eon, 4 to 4.56 billion years ago, are exceedingly rare, found in only 12 places on Earth, and usually in numbers fewer than three at each location.

Hafnium isotopes and trace elements preserved in the Greenstone Belt zircons told a story about the conditions on Earth at the time they crystalized. Zircons 3.8-billion-years-old and younger appeared to have formed in rock experiencing pressures and melting similar to modern subduction zones, suggesting the crust may have started moving.

"When I say plate tectonics, I'm specifically referring to an arc setting, when one plate goes under another and you have all that volcanism -- think of the Andes, for example, and the Ring of Fire," Drabon said, describing a classic example of subduction.

"At 3.8 billion years there is a dramatic shift where the crust is destabilized, we have new rocks forming and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics," Drabon said.

In contrast, the older zircons preserved evidence of a global cap of "protocrust" derived from remelting mantle rock that had remained stable for 600 million years, the study found.

Signs of global change

The new study found a similar transition to conditions resembling modern subduction in zircons from other locations around the world, dating to within about 200 million years of the South African zircons.

"We see evidence for a significant change on the Earth around 3.8 to 3.6 billion years ago and evolution toward plate tectonics is one clear possibility." Drabon said.

While not conclusive, the results suggest a global change may have begun, Drabon said, possibly starting and stopping in scattered locations before settling into the efficient global engine of constantly moving plates we see today.

Plate tectonics shapes Earth's atmosphere as well as its surface. Release of volcanic gasses and production of new silicate rock, which consumes large amounts carbon dioxide from the atmosphere, temper large temperature swings from too much or too little greenhouse gas.

"Without all of the recycling and new crust forming, we might be going back and forth between boiling hot and freezing cold," Drabon said. "It's kind of like a thermostat for the climate."

Plate tectonics has, so far, only been observed on Earth, and may be essential to making a planet livable, Drabon said, which makes the origins of plate motions of interest in research into the early development of life.

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Large, long-lived, and entirely molten magma chambers once existed in Earth’s crust

An international group of researchers led by geologists from Wits University in Johannesburg have come up with multiple lines of evidence indicating that the Bushveld Complex in South Africa functioned as a "big magma tank" in the ancient Earth's crust. This research was published as a paper in Scientific Reports.

Professor Rais Latypov from the School of Geosciences at Wits University says "While re-examining thin-sections of Bushveld chromitites, we noticed a very puzzling observation: chromite often occurs as individual grains that seemingly 'suspended' within matrix minerals. This observation leads us to a critical question: why have the chromite grains failed to sink towards the chamber floor despite being much denser than the host melt?"

To answer this question, the researchers have studied chromitite in three-dimensions (3D) using high-resolution X-ray computed tomography and revealed that nearly all chromite grains are closely interconnected to form a single continuous 3D framework. "This gave us an answer to the above question: chromite grains are not able to settle freely towards the chamber floor simply because they are all bound together in self-supporting 3D frameworks attached to the chamber floor," says Dr Sofya Chistyakova from the School of Geosciences at Wits University.

There is only one process that may result in the formation of such 3D frameworks of chromite crystals. This is an in situ self-nucleation and growth of chromite grains, for example, when all new chromite grains nucleate and grow on pre-existing chromite grains directly at the chamber floor. This happens from the parental melt that is saturated in chromite as the only crystallising phase.

"This logically brought us to a long-known Cr mass balance issue -- normal basaltic melts contain only a very small amount of Cr so that the formation of thick chromitite layer requires extraction of Cr from a very large volume of liquid that must be present as a thick melt layer in the chamber. Simple mass balance calculations indicate that a 1 metre thick layer of chromitite will require a magma column of 2km to 4km thick," says Latypov.

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Tilting of Earth’s crust governed the flow of ancient megafloods

As ice sheets began melting at the end of the last ice age, a series of cataclysmic floods called the Missoula megafloods scoured the landscape of eastern Washington, carving long, deep channels and towering cliffs through an area now known as the Channeled Scablands. They were among the largest known floods in Earth’s history, and geologists struggling to reconstruct them have now identified a crucial factor governing their flows.

In a study published February 14 in Proceedings of the National Academy of Sciences, researchers showed how the changing weight of the ice sheets would have caused the entire landscape to tilt, changing the course of the megafloods.

“People have been looking at high water marks and trying to reconstruct the size of these floods, but all of the estimates are based on looking at the present-day topography,” said lead author Tamara Pico, assistant professor of Earth and planetary sciences at UC Santa Cruz. “This paper shows that the ice age topography would have been different over broad scales due to the deformation of Earth’s crust by the weight of the ice sheets.”

During the height of the last ice age, vast ice sheets covered much of North America. They began to melt after about 20,000 years ago, and the Missoula megafloods occurred between 18,000 and 15,500 years ago. Pico’s team studied how the changing weight of the ice sheets during this period would have tilted the topography of eastern Washington, changing how much water would flow into different channels during the floods.

Glacial Lake Missoula formed in western Montana when a lobe of the Cordilleran ice sheet dammed the Clark Fork valley in the Idaho panhandle and melt water built up behind the dam. Eventually the water got so deep that the ice dam began to float, resulting in a glacial outburst flood. After enough water had been released, the ice dam resettled and the lake refilled. This process is thought to have been repeated dozens of times over a period of several thousand years.

Downstream from glacial Lake Missoula, the Columbia River was dammed by another ice lobe, forming glacial Lake Columbia. When Lake Missoula’s outburst floods poured into Lake Columbia, the water spilled over to the south onto the eastern Washington plateau, eroding the landscape and creating the Channeled Scablands.

During this period, the deformation of the Earth’s crust in response to the growing and shrinking of ice sheets would have changed the elevation of the topography by hundreds of meters, Pico said. Her team incorporated these changes into flood models to investigate how the tilting of the landscape would have changed the routing of the megafloods and their erosional power in different channels.

“We used flood models to predict the velocity of the water and the erosional power in each channel, and compared that to what would be needed to erode basalt, the type of rock on that landscape,” Pico said.

They focused on two major channel systems, the Cheney-Palouse and Telford-Crab Creek tracts. Their results showed that earlier floods would have eroded both tracts, but that in later floods the flow would have been concentrated in the Telford-Crab Creek system.

“As the landscape tilted, it affected both where the water overflowed out of Lake Columbia and how water flowed in the channels, but the most important effect was on the spillover into those two tracts,” Pico said. “What’s intriguing is that the topography isn’t static, so we can’t just look at the topography of today to reconstruct the past.”

The findings provide a new perspective on this fascinating landscape, she said. Steep canyons hundreds of feet deep, dry falls, and giant potholes and ripple marks are among the many remarkable features etched into the landscape by the massive floods.

“When you are there in person, it’s crazy to think about the scale of the floods needed to carve those canyons, which are now dry,” Pico said. “There are also huge dry waterfalls—it’s a very striking landscape.”

She also noted that the oral histories of Native American tribes in this region include references to massive floods. “Scientists were not the first people to look at this,” Pico said. “People may even have been there to witness these floods.”

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Contorted oceanic plate caused complex quake off New Zealand’s East Cape

Subduction zones, where a slab of oceanic plate is pushed beneath another tectonic plate down into the mantle, cause the world's largest and most destructive earthquakes. Reconstructing the geometry and stress conditions of the subducted slabs at subduction zones is crucial to understanding and preparing for major earthquakes. However, the tremendous depths of these slabs make this challenging -- seismologists rely mainly on the rare windows into these deeply buried slabs provided by the infrequent but strong earthquakes, termed intraslab earthquakes, that occur within them.

In a new study published in Geophysical Research Letters, a research team led by the University of Tsukuba used seismic data generated by a magnitude 7.3 earthquake that occurred off the northeasternmost tip of New Zealand's North Island on March 4, 2021, detected by seismometers around the world, to investigate the particularly unusual geometry and stress states of the subducted slab deep below the surface in this region.

"The 2021 East Cape earthquake showed a complex rupture process, likely because of its location at the boundary between the Kermadec Trench to the north and the Hikurangi Margin to the south," lead author of the study Assistant Professor Ryo Okuwaki explains. "To investigate the geometry of the stress field and earthquake rupture process, we used a novel finite-fault inversion technique that required no pre-existing knowledge of the area's faults."

This investigation revealed multiple episodes of rupture, generated by both compression and extension in the subsurface at different depths. These episodes included shallow (~30 km) rupture due to extension perpendicular to the trench as would typically be expected in a subduction zone. Unexpectedly, however, the deep (~70 km) rupture occurred with compression parallel to the subduction trench.

"Two alternative or inter-related factors may explain the unique rupture geometry of the 2021 East Cape earthquake," senior author Professor Yuji Yagi explains. "First, subduction of a seamount or multiple seamounts along with the subducted slab could contort the slab and create local changes in the stress field. Second, the transition from the Kermadec Trench to the Hikurangi Margin, where the subducted oceanic crust is considerably thicker, could create the local conditions responsible for the unusual faulting pattern."

Because of the rarity of deep intraslab earthquakes in this region, distinguishing between these two possibilities is currently challenging, and indeed both factors might play significant roles in creating the complex stress field revealed by the East Cape earthquake. Additional earthquakes off the northeast coast of New Zealand in the future may shed further light on this deep tectonic mystery.

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What control the height of mountains? Surprisingly, it is not erosion

Which forces and mechanisms determine the height of mountains? A group of researchers from Münster and Potsdam has now found a surprising answer: It is not erosion and weathering of rocks that determine the upper limit of mountain massifs, but rather an equilibrium of forces in the Earth's crust. This is a fundamentally new and important finding for the earth sciences. The researchers report on it in the scientific journal Nature.

The highest mountain ranges on Earth -- such as the Himalayas or the Andes -- arise along convergent plate boundaries. At such plate boundaries two tectonic plates move toward each other, and one of the plates is forced beneath the other into the Earth's mantle. During this process of subduction, strong earthquakes repeatedly occur on the plate interface, and over millions of years mountain ranges are built at the edges of the continents.

Whether the height of mountain ranges is mainly determined by tectonic processes in the Earth's interior or by erosional processes sculpturing the Earth's surface has long been debated in geosciences.

A new study led by Armin Dielforder of GFZ German Research Centre for Geoscience now shows that erosion by rivers and glaciers has no significant influence on the height of mountain ranges. Together with scientists from the GFZ and the University of Münster (Germany), he resolved the longstanding debate by analysing the strength of various plate boundaries and calculating the forces acting along the plate interfaces.

The researchers arrived at this surprising result by calculating the forces along different plate boundaries on the Earth. They used data that provide information about the strength of plate boundaries. These data are derived, for example, from heat flow measurements in the subsurface. The heat flow at convergent plate boundaries is in turn influenced by the frictional energy at the interfaces of the continental plates.

One can imagine the formation of mountains using a tablecloth. If you place both hands under the cloth on the table top and push it, the cloth folds and at the same time it slides a little over the back of your hands. The emerging folds would correspond, for instance, to the Andes, the sliding over the back of the hands to the friction in the underground. Depending on the characteristics of the rock, tensions also build up in the deep underground which are discharged in severe earthquakes, especially in subduction zones.

The researchers collected worldwide data from the literature on friction in the subsurface of mountain ranges of different heights (Himalayas, Andes, Sumatra, Japan) and calculated the resulting stress and thus the forces that lead to the uplift of the respective mountains. In this way they showed that in active mountains the force on the plate boundary and the forces resulting from the weight and height of the mountains are in balance.

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New findings on Earth's magnetic field

Inside the Earth concept illustration.

The huge magnetic field which surrounds the Earth, protecting it from radiation and charged particles from space -- and which many animals even use for orientation purposes -- is changing constantly, which is why geoscientists keep it constantly under surveillance. The old well-known sources of the Earth's magnetic field are the Earth's core -- down to 6,000 kilometres deep down inside the Earth -- and the Earth's crust: in other words, the ground we stand on. The Earth's mantle, on the other hand, stretching from 35 to 2,900 kilometres below the Earth's surface, has so far largely been regarded as "magnetically dead." An international team of researchers from Germany, France, Denmark and the USA has now demonstrated that a form of iron oxide, hematite, can retain its magnetic properties even deep down in the Earth's mantle. This occurs in relatively cold tectonic plates, called slabs, which are found especially beneath the western Pacific Ocean.

"This new knowledge about the Earth's mantle and the strongly magnetic region in the western Pacific could throw new light on any observations of the Earth's magnetic field," says mineral physicist and first author Dr. Ilya Kupenko from the University of Münster (Germany). The new findings could, for example, be relevant for any future observations of the magnetic anomalies on the Earth and on other planets such as Mars. This is because Mars has no longer a dynamo and thus no source enabling a strong magnetic field originating from the core to be built up such as that on Earth. It might, therefore, now be worth taking a more detailed look on its mantle. The study has been published in the "Nature" journal.

Background and methods used:

Deep in the metallic core of the Earth, it is liquid iron alloy that triggers electrical flows. In the outermost crust of the Earth, rocks cause magnetic signal. In the deeper regions of the Earth's interior, however, it was believed that the rocks lose their magnetic properties due to the very high temperatures and pressures.

The researchers now took a closer look at the main potential sources for magnetism in the Earth's mantle: iron oxides, which have a high critical temperature -- i.e. the temperature above which material is no longer magnetic. In the Earth's mantle, iron oxides occur in slabs that are buried from the Earth's crust further into the mantle, as a result of tectonic shifts, a process called subduction. They can reach a depth within the Earth's interior of between 410 and 660 kilometres -- the so-called transition zone between the upper and the lower mantle of the Earth. Previously, however, no one had succeeded in measuring the magnetic properties of the iron oxides at the extreme conditions of pressure and temperature found in this region.

Now the scientists combined two methods. Using a so-called diamond anvil cell, they squeezed micrometric-sized samples of iron oxide hematite between two diamonds, and heated them with lasers to reach pressures of up to 90 gigapascal and temperatures of over 1,000 °C (1,300 K). The researchers combined this method with so-called Mössbauer spectroscopy to probe the magnetic state of the samples by means of synchrotron radiation. This part of the study was carried out at the ESRF synchrotron facility in Grenoble, France, and this made it possible to observe the changes of the magnetic order in iron oxide.

The surprising result was that the hematite remained magnetic up to a temperature of around 925 °C (1,200 K) -- the temperature prevailing in the subducted slabs beneath the western part of Pacific Ocean at the Earth's transition zone depth. "As a result, we are able to demonstrate that the Earth's mantle is not nearly as magnetically 'dead' as has so far been assumed," says Prof. Carmen Sanchez-Valle from the Institute of Mineralogy at Münster University. "These findings might justify other conclusions relating to the Earth's entire magnetic field," she adds.

Relevance for investigations of the Earth's magnetic field and the movement of the poles

By using satellites and studying rocks, researchers observe the Earth's magnetic field, as well as the local and regional changes in magnetic strength. Background: The geomagnetic poles of the Earth -- not to be confused with the geographic poles -- are constantly moving. As a result of this movement they have actually changed positions with each other every 200,000 to 300,000 years in the recent history of the Earth. The last poles flip happened 780,000 years ago, and last decades scientists report acceleration in the movement of the Earth magnetic poles. Flip of magnetic poles would have profound effect on modern human civilisation. Factors which control movements and flip of the magnetic poles, as well as directions they follow during overturn are not understood yet.

One of the poles' routes observed during the flips runs over the western Pacific, corresponding very noticeably to the proposed electromagnetic sources in the Earth's mantle. The researchers are therefore considering the possibility that the magnetic fields observed in the Pacific with the aid of rock records do not represent the migration route of the poles measured on the Earth's surface, but originate from the hitherto unknown electromagnetic source of hematite-containing rocks in the Earth's mantle beneath the West Pacific.

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Earth recycles ocean floor into diamonds

Diamond and coal

The diamond on your finger is most likely made of recycled seabed cooked deep in the Earth.

Traces of salt trapped in many diamonds show the stones are formed from ancient seabeds that became buried deep beneath the Earth's crust, according to new research led by Macquarie University geoscientists in Sydney, Australia.

Most diamonds found at the Earth's surface are formed this way; others are created by crystallization of melts deep in the mantle.

In experiments recreating the extreme pressures and temperatures found 200 kilometres underground, Dr Michael Förster, Professor Stephen Foley, Dr Olivier Alard, and colleagues at Goethe Universität and Johannes Gutenberg Universität in Germany, have demonstrated that seawater in sediment from the bottom of the ocean reacts in the right way to produce the balance of salts found in diamond.

The study, published in Science Advances, settles a long-standing question about the formation of diamonds. "There was a theory that the salts trapped inside diamonds came from marine seawater, but couldn't be tested," says lead author Michael. "Our research showed that they came from marine sediment."

Diamonds are crystals of carbon that form beneath the Earth's crust in very old parts of the mantle. They are brought to the surface in volcanic eruptions of a special kind of magma called kimberlite.

While gem diamonds are usually made of pure carbon, so-called fibrous diamonds, which are cloudy and less appealing to jewellers, often include small traces of sodium, potassium and other minerals that reveal information about the environment where they formed.

These fibrous diamonds are commonly ground down and used in technical applications like drill bits.

Fibrous diamonds grow more quickly than gem diamonds, which means they trap tiny samples of fluids around them while they form.

"We knew that some sort of salty fluid must be around while the diamonds are growing, and now we have confirmed that marine sediment fits the bill," says Michael.

For this process to occur, a large slab of sea floor would have to slip down to a depth of more than 200 kilometres below the surface quite rapidly, in a process known as subduction in which one tectonic plate slides beneath another.

The rapid descent is required because the sediment must be compressed to more than four gigapascals (40,000 times atmospheric pressure) before it begins to melt in the temperatures of more than 800°C found in the ancient mantle.

To test the idea, team members at the Johannes Gutenberg Universität Mainz and Goethe Universität Frankfurt in Germany carried out a series of high-pressure, high-temperature experiments.

They placed marine sediment samples in a vessel with a rock called peridotite that is the most common kind of rock found in the part of the mantle where diamonds form. Then they turned up the pressure and the heat, giving the samples time to react with one another in conditions like those found at different places in the mantle.

At pressures between four and six gigapascals and temperatures between 800°C and 1100°C, corresponding to depths of between 120 and 180 kilometres below the surface, they found salts formed with a balance of sodium and potassium that closely matches the small traces found in diamonds.

"We demonstrated that the processes that lead to diamond growth are driven by the recycling of oceanic sediments in subduction zones," says Michael.

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Data mining digs up hidden clues to major California earthquake triggers

A historic image of quake damage in Long Beach, California, 1933.

A powerful computational study of southern California seismic records has revealed detailed information about a plethora of previously undetected small earthquakes, giving a more precise picture about stress in the earth's crust. A new publicly available catalog of these findings will help seismologists better understand the stresses triggering the larger earthquakes that occasionally rock the region.

"It's very difficult to unpack what triggers larger earthquakes because they are infrequent, but with this new information about a huge number of small earthquakes, we can see how stress evolves in fault systems," said Daniel Trugman, a post-doctoral fellow at Los Alamos National Laboratory and coauthor of a paper published in the journal Science today. "This new information about triggering mechanisms and hidden foreshocks gives us a much better platform for explaining how big quakes get started," Trugman said.

Crunching the Numbers

Trugman and coauthors from the California Institute of Technology and Scripps Institution of Oceanography performed a massive data mining operation of the Southern California Seismic Network for real quakes buried in the noise. The team was able to detect, understand, and locate quakes more precisely, and they created the most comprehensive earthquake catalog to date. The work identified 1.81 million quakes -- 10 times more earthquakes occurring 10 times more frequently than quakes previously identified using traditional seismology methods.

The team developed a comprehensive, detailed earthquake library for the entire southern California region, called the Quake Template Matching (QTM) catalog. They are using it to create a more complete map of California earthquake faults and behavior. This catalog may help researchers detect and locate quakes more precisely.

The team analyzed nearly two decades of data collected by the Southern California Seismic Network. The network, considered one of the world's best seismic systems, amasses a catalog of quakes from 550 seismic monitoring stations in the region. The SCSN catalog is based entirely on the traditional approach: manual observation and visual analysis. But Trugman says this traditional approach misses many weak signals that are indicators of small earthquakes.

Matching Templates Is Key

The team improved on this catalog with data mining. Using parallel computing, they crunched nearly 100 terabytes of data across 200 graphics processing units. Zooming in at high resolution for a 10-year period, they performed template matching using seismograms (waveforms or signals) of previously identified quakes. To create templates, they cut out pieces of waveforms from previously recorded earthquakes and matched those waveforms to patterns of signals recorded simultaneously from multiple seismic stations. Template matching has been done before, but never at this scale.

"Now we can automate it and search exhaustively through the full waveform archive to find signals of very small earthquakes previously hidden in the noise," Trugman explained.

Applying the templates found events quake precursors, foreshocks and small quakes that had been missed with manual methods. Those events often provide key physical and geographic details to help predict big quakes. The team also identified initiation sequences that reveal how quakes are triggered.

New details also revealed three-dimensional geometry and fault structures, which will support development of more realistic models.

Recently, Trugman and Los Alamos colleagues have applied machine learning to study earthquakes created in laboratory quake machines. That works has uncovered important details about earthquake behavior that may be used to predict quakes.

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Massive Bolivian earthquake reveals mountains 660 kilometers below our feet

Graphic showing the Transition Zone inside the Earth Princeton seismologist Jessica Irving worked with then-graduate student Wenbo Wu and another collaborator to determine the roughness at the top and bottom of the transition zone, a layer within the mantle, using scattered earthquake waves. They found that the top of the transition zone, a layer located 410 kilometers down, is mostly smooth, but the base of the transition zone, 660 km down, in some places is much rougher than the global surface average. “In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary,” said Wu. NOTE: This graphic is not to scale.

Most schoolchildren learn that the Earth has three (or four) layers: a crust, mantle and core, which is sometimes subdivided into an inner and outer core. That's not wrong, but it does leave out several other layers that scientists have identified within the Earth.

In a study published this week in Science, Princeton geophysicists Jessica Irving and Wenbo Wu, in collaboration with Sidao Ni from the Institute of Geodesy and Geophysics in China, used data from an enormous earthquake in Bolivia to find mountains and other topography on a layer located 660 kilometers (410 miles) straight down, which separates the upper and lower mantle. (Lacking a formal name for this layer, the researchers simply call it "the 660-km boundary.")

To peer deep into the Earth, scientists use the most powerful waves on the planet, which are generated by massive earthquakes. "You want a big, deep earthquake to get the whole planet to shake," said Irving, an assistant professor of geosciences.

Big earthquakes are vastly more powerful than small ones -- energy increases 30-fold with every step up the Richter scale -- and deep earthquakes, "instead of frittering away their energy in the crust, can get the whole mantle going," Irving said. She gets her best data from earthquakes that are magnitude 7.0 or higher, she said, as the shockwaves they send out in all directions can travel through the core to the other side of the planet -- and back again. For this study, the key data came from waves picked up after a magnitude 8.2 earthquake -- the second-largest deep earthquake ever recorded -- that shook Bolivia in 1994.

"Earthquakes this big don't come along very often," she said. "We're lucky now that we have so many more seismometers than we did even 20 years ago. Seismology is a different field than it was 20 years ago, between instruments and computational resources."

Seismologists and data scientists use powerful computers, including Princeton's Tiger supercomputer cluster, to simulate the complicated behavior of scattering waves in the deep Earth.

The technology depends on a fundamental property of waves: their ability to bend and bounce. Just as light waves can bounce (reflect) off a mirror or bend (refract) when passing through a prism, earthquake waves travel straight through homogenous rocks but reflect or refract when they encounter any boundary or roughness.

"We know that almost all objects have surface roughness and therefore scatter light," said Wu, the lead author on the new paper, who just completed his geosciences Ph.D. and is now a postdoctoral researcher at the California Institute of Technology. "That's why we can see these objects -- the scattering waves carry the information about the surface's roughness. In this study, we investigated scattered seismic waves traveling inside the Earth to constrain the roughness of the Earth's 660-km boundary."

The researchers were surprised by just how rough that boundary is -- rougher than the surface layer that we all live on. "In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary," said Wu. Their statistical model didn't allow for precise height determinations, but there's a chance that these mountains are bigger than anything on the surface of the Earth. The roughness wasn't equally distributed, either; just as the crust's surface has smooth ocean floors and massive mountains, the 660-km boundary has rough areas and smooth patches. The researchers also examined a layer 410 kilometers (255 miles) down, at the top of the mid-mantle "transition zone," and they did not find similar roughness.

"They find that Earth's deep layers are just as complicated as what we observe at the surface," said seismologist Christine Houser, an assistant professor at the Tokyo Institute of Technology who was not involved in this research. "To find 2-mile (1-3 km) elevation changes on a boundary that is over 400 miles (660 km) deep using waves that travel through the entire Earth and back is an inspiring feat. ... Their findings suggest that as earthquakes occur and seismic instruments become more sophisticated and expand into new areas, we will continue to detect new small-scale signals which reveal new properties of Earth's layers."

What it means

The presence of roughness on the 660-km boundary has significant implications for understanding how our planet formed and continues to function. That layer divides the mantle, which makes up about 84 percent of the Earth's volume, into its upper and lower sections. For years, geoscientists have debated just how important that boundary is. In particular, they have investigated how heat travels through the mantle -- whether hot rocks are carried smoothly from the core-mantle boundary (almost 2,000 miles down) all the way up to the top of the mantle, or whether that transfer is interrupted at this layer. Some geochemical and mineralogical evidence suggests that the upper and lower mantle are chemically different, which supports the idea that the two sections don't mix thermally or physically. Other observations suggest no chemical difference between the upper and lower mantle, leading some to argue for what's called a "well-mixed mantle," with both the upper and lower mantle participating in the same heat-transfer cycle.

"Our findings provide insight into this question," said Wu. Their data suggests that both groups might be partially right. The smoother areas of the 660-km boundary could result from more thorough vertical mixing, while the rougher, mountainous areas may have formed where the upper and lower mantle don't mix as well.

In addition, the roughness the researchers found, which existed at large, moderate and small scales, could theoretically be caused by heat anomalies or chemical heterogeneities. But because of how heat in transported within the mantle, Wu explained, any small-scale thermal anomaly would be smoothed out within a million years. That leaves only chemical differences to explain the small-scale roughness they found.

What could cause significant chemical differences? The introduction of rocks that used to belong to the crust, now resting quietly in the mantle. Scientists have long debated the fate of the slabs of sea floor that get pushed into the mantle at subduction zones, the collisions happening found all around the Pacific Ocean and elsewhere around the world. Wu and Irving suggest that remnants of these slabs may now be just above or just below the 660-km boundary.

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Earth's continental nurseries discovered beneath mountains

The central Andes Mountains and surrounding landscape, as seen in this true-color image from NASA’s Terra spacecraft, formed over the past 170 million years as the Nazca Plate lying under the Pacific Ocean has forced its way under the South American Plate.

In his free time last summer, Rice University geoscientist Ming Tang made a habit of comparing the niobium content in various rocks in a global minerals database. What he found was worth skipping a few nights out with friends.

In a paper published this month by Nature Communications, Tang, Rice petrologist Cin-Ty Lee and colleagues offered an answer to one of Earth science's fundamental questions: Where do continents form?

"If our conclusions are correct, every piece of land that we are now sitting on got its start someplace like the Andes or Tibet, with very mountainous surfaces," said Tang, lead author of the study and a postdoctoral research associate in Rice's Department of Earth, Environmental and Planetary Sciences (EEPS). "Today, most places are flat because that is the stable stage of the continental crust. But what we found was that when the crust formed, it had to start out with mountain-building processes."

The connection between niobium, one of Earth's rarest elements, and continent formation is a story that plays out over billions of years at scales as small as molecules and as large as mountain ranges. The leading players are niobium and tantalum, rare metals so alike that geologists often think of them as twins.

"They have very similar chemical properties and behave almost identically in most geological processes," Tang said. "If you measure tantalum and niobium, you find that their ratio is nearly constant in Earth's mantle. That means that when you find more niobium in a rock, you will find more tantalum, and when you find less niobium, you will find less tantalum."

The mantle is Earth's thickest layer, spanning about 1,800 miles between the planet's core and its thin outer crust. Earth scientists believe that little, if anything, moves between the mantle and core, but the mantle and everything above it -- seafloor, oceans, continents and atmosphere -- are connected, and many of the atoms on Earth's surface today, including the atoms in humans and other living things, have cycled through the mantle one or more times in Earth's 4.6 billion years.

The rocks in continents are an exception. Geologists have found some that are up to 4 billion years old, which means they were formed near the surface and stayed on the surface, without being recycled into the mantle. That's due in part to the nature of continental crust, which is far less dense than the basaltic rocks beneath Earth's oceans. Lee, professor and EEPS department chair, said it's no coincidence that Earth is the only rocky planet known to have both continents and life.

"Every day we live on continents, and we take most of our resources from continents," Lee said. "We have oxygen in the air to breath and just the right temperature to support complex life. These things are so common that we take them for granted, but Earth didn't start off with these conditions. They developed later in Earth's history. And the emergence of continents is one of the things that shaped our planet and made it more livable."

Scientists still lack details about how continents got their start and how they grew to cover 30 percent of Earth's surface, but one big clue relates to niobium and tantalum, the geochemical twins.

"On average, the rocks in continental crust have about 20 percent less niobium than they should compared to the rock we see everywhere else," Tang said. "We believe this missing niobium is tied to the mystery of continents. By solving or finding the missing the niobium, we can get important information about how continents form."

Geologists have known about the imbalance for decades. And it certainly suggests that the geochemical processes that produce continental crust also remove niobium. But where was the missing niobium?

That nagging question prompted Tang to spend his free time perusing records in the Max Planck Institute's GEOROC database, a comprehensive global collection of published analyses of volcanic rocks.

Based on those searches and months of follow-up tests, Tang, Lee and colleagues offer the first physical evidence that "arclogites" (pronounced ARC-loh-jyts) are responsible for the missing niobium. Arclogites are cumulates, the leftover dross that accumulates near the base of continental arcs. On rare occasions, chunks of these cumulates erupt onto the surface from volcanos.

The Rice group first sent arclogite samples that Lee had collected in Arizona to their collaborator, Kang Chen, a research fellow based at the China University of Geosciences in Wuhan. Chen spent a month getting precise readings of the relative amounts of niobium and tantalum in the samples. The rocks were created when the High Sierras were an active continental arc, like the Andes today.

Chen's tests confirmed high niobium-tantalum ratios, but to better understand the mechanism by which this signature was developed, Tang and Lee used high precision laser ablation and "inductively coupled plasma mass spectrometry" in Lee's laboratory at Rice to reveal the mineral rutile was responsible.

"Rutile is the mineral that hosts the niobium," he said. "It's a naturally occurring form of titanium oxide, and it is what actually 'sees' the difference between niobium and tantalum and captures one more than the other."

But that happens only under specific conditions. For example, Tang said that at temperatures above 1,000 degrees Celsius, rutile traps normal ratios of tantalum and niobium. It only begins to prefer niobium when temperatures drop below 1,000 degrees Celsius. Tang said the only known place with that set of conditions is deep beneath continental arcs, like the Andes today or the High Sierras about 80 million years ago.

"The reason you need high pressure is that titanium oxide is relatively rare," he said. "You need very high pressure to force it to crystalize and fall out of the magma."

In an earlier arclogite study published in Science Advances last May, Tang and Lee discovered a subtle chemical signature that can explain why continental crust is iron-depleted. Lee said that finding and the discovery about rutile and niobium illustrate the central importance of continental arcs in Earth history.

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Earth’s inner core is solid, 'J waves' suggest

View of Earth from space, showing North Africa, Europe and the Middle East. Elements of this image furnished by NASA.

A new study by researchers at The Australian National University (ANU) could help us understand how our planet was formed.

Associate Professor Hrvoje Tkalčić and PhD Scholar Than-Son Phạm are confident they now have direct proof that Earth's inner core is solid.

They came up with a way to detect shear waves, or "J waves" in the inner core -- a type of wave which can only travel through solid objects.

"We found the inner core is indeed solid, but we also found that it's softer than previously thought," Associate Professor Tkalčić said.

"It turns out -- if our results are correct -- the inner core shares some similar elastic properties with gold and platinum. The inner core is like a time capsule, if we understand it we'll understand how the planet was formed, and how it evolves."

Inner core shear waves are so tiny and feeble they can't be observed directly. In fact, detecting them has been considered the "Holy Grail" of global seismology since scientists first predicted the inner core was solid in the 1930s and 40s.

So the researchers had to come up with a creative approach.

Their so-called correlation wavefield method looks at the similarities between the signals at two receivers after a major earthquake, rather than the direct wave arrivals. A similar technique has been used by the same team to measure the thickness of the ice in Antarctica.

"We're throwing away the first three hours of the seismogram and what we're looking at is between three and 10 hours after a large earthquake happens. We want to get rid of the big signals," Dr Tkalčic said.

"Using a global network of stations, we take every single receiver pair and every single large earthquake -- that's many combinations -- and we measure the similarity between the seismograms. That's called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram -- a sort of fingerprint of the Earth."

The study shows these results can then be used to demonstrate the existence of J waves and infer the shear wave speed in the inner core.

While this specific information about shear waves is important, Dr Tkalčić says what this research tells us about the inner core is even more exciting.

"For instance we don't know yet what the exact temperature of the inner core is, what the age of the inner core is, or how quickly it solidifies, but with these new advances in global seismology, we are slowly getting there.

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New insight into Earth's crust, mantle and outer core interactions

Earth's crust, mantle and outer core interactions.

A new study by the University of Liverpool, in collaboration with the Universities of Lancaster and Oslo, sheds light on a longstanding question that has puzzled earth scientists.

Using previously unavailable data, researchers confirm a correlation between the movement of plate tectonics on the Earth's surface, the flow of mantle above the Earth's core and the rate of reversal of the Earth's magnetic field which has long been hypothesised.

In a paper published in the journal Tectonophysics, they suggest that it takes around 120-130 million years for slabs of ancient ocean floor to sink (subduct) from the Earth's surface to a sufficient depth in the mantle where they can cool the core, which in turn causes the liquid iron in the Earth's outer core to flow more vigorously and produce more reversals of the Earth's magnetic field.

This study is the first to demonstrate this correlation using records and proxies of global rates of subduction from various sources including a continuous global plate reconstruction model developed at the University of Sydney. These records were compared with a new compilation of magnetic field reversals whose occurrence is locked into volcanic and sedimentary rocks.

Liverpool palaeomagnetist, Professor Andy Biggin, said: "Until recently we did not have good enough records of how much global rates of subduction had changed over the last few hundreds of millions of years and so we had nothing to compare with the magnetic records.

"When we were able to compare them, we found that the two records of subduction and magnetic reversal rate do appear to be correlated after allowing for a time delay of 120-130 million years for the slabs of ocean floor to go from the surface to a sufficient depth in the mantle where they can cool the core.

"We do not know for sure that the correlation is causal but it does seem to fit with our understanding of how the crust, mantle and core should all be interacting and this value of 120-130 million could provide a really useful observational constraint on how quickly slabs of ancient sea floor can fall through the mantle and affect flow currents within it and in the underlying core."

The magnetic field is generated deep within the Earth in a fluid outer core of iron and other elements that creates electric currents, which in turn produces magnetic fields.

The core is surrounded by a nearly 3,000 km thick mantle which although made of solid rock, flows very slowly (mm per year). The mantle produces convection currents which are strongly linked to movement of the tectonic plates but also affect the core by varying the amount of heat that is transferred across the core-mantle boundary.

The Earth's magnetic field occasionally flips its polarity and the average length of time between such flips has changed dramatically through Earth's history. For example, today such magnetic reversals occur on average four times per million years but one hundred million years ago, the field essentially stayed in the same polarity for nearly 40 million years.

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Earth’s Crust Absorbs Lots of Carbon Dioxide, But Not Enough to Save Humanity

The carbon cycle of Earth's oceans

The seafloor is absorbing carbon dioxide, the greenhouse gas most associated with climate change, according to researchers at the University of Sydney in Australia. But while the ocean bottom might someday help reverse global warming, the process would take millions of years.

The authors of the study published recently in the journal Science Advances said their work provides a sense of scale for the damage humankind is wreaking on the planet today.

“We are starting to alter the environment — the surface conditions of the planet — a lot. It’s a fair question to ask, ‘Where will this ultimately lead?’” study co-author Dietmar Müller, a geophysicist. “In the very long run, we’re heating up the planet. Perhaps all ice will be melted one day. What safeguards has the planet built in? Ultimately, it will start cooling again. That will maintain habitability of the Earth in the very long run and make sure not all life will become extinct.”

With funding from the Australian Research Council and the Alfred P. Sloan Foundation, Müller and his colleague, sedimentologist Adriana Dutkiewicz, studied decades’ worth of drilling samples from the ocean floor. Using computer models that took into account changing water temperatures, they determined how much carbon the seabed absorbed.

They found that the ocean floor could absorb as much as 22 million tons of carbon annually.

That might sound like an enormous number, but in relative terms, it’s not. The planet’s atmosphere contains almost 950 billion tons of carbon. Humans dump around 37 billion tons of carbon into the atmosphere annually, according to the Global Carbon Project.

“This research is not about looking at short time scales,” said Müller.

Carbon sinks through subduction, or the process of tectonic plates on the earth’s crust moving and buckling under one another into the Earth’s mantle, the zone of magma and rock that is between the planet’s outer layer and core. As the plates under the water move over the eons, they rupture the Earth’s crust under the sea, forming fissures that expose minerals like calcite that capture carbon and turn into concrete-like materials. The warmer the water, the more efficient this chemical reaction that occurs.

The carbon doesn’t necessarily stay in the ocean crust forever. Instead, it’s recycled. Volcanos that often form along the ridges of subduction zones — like the so-called Ring of Fire along the rim of the Pacific Ocean — later spew up some of that carbon.

The researchers found that the capacity of the ocean to handle carbon runs in cycles of 26 million years. The planet is currently in an expansionary cycle, said Müller. It’s not clear why the crust operates according to this schedule, but geological records of fossils, salt deposits and other phenomena also follow patterns along similar time periods, he said.

Read more at Seeker